Extractive distillation of aromatics



Oct. 23, 1956 Y E. H. LEBEIS Er'AL 2,768,131

EXTRACTIVE DISTILLATION oF Amon/Mmes- Filed nec. 19, 1952 INI/ENTOR.

l 'Ehm/HM@ gj HTORNEY nited States Patent 2,768,131 EXTRACTIVE DISTILLATION OF AROMATICS Edward `II. Lebeis, Media, Pa., and Frank 'S. Bondr,

Wilmington, Del., assgnors to Houdry Process Corporation, Wilmington, Del., a corporation of Delaware Application December 19, 195:2, serial No. 326,962 6 claims. (cl.- 2oz- 39.55

The present invention relates generally to the separation,- concentration and/or puriiication of aromatic hydrocarbons and is particularly concerned with improvements and methods for the recovery of mono-nuclear aromatic hydrocarbons `such as benzene `and toluene from their admixtures with non-aromatic hydrocarbon `components in a naphtha fraction. Y

Hydrocarbon distillate's boiling in the range of gasoline' and naphtha from various sources `and particularly such distillates obtained by 'catalytic reforming of naphtha comprise aromatic hydrocarbons .in admixture with naphthenes and parains having boiling points very `close to those of the `aromatic components of the distillate, which renders the separation of the aromatic components by ordinary methods of fractional distillation extremely diiicult or impossible. Moreover, certain of these components may form-'azeotropes withv the aromatic hydrocarbon desired to be separated. Among the various separation techniques which have been tried with varying degrees of success there are includedf liquid solvent extraction, selective azeotropic distillation, selective chromatographic adsorption, and extractive clistillation.v The present invention is concerned with improvements processes involving Ithe last named type' of operation, which type of process offers' considerable flexibility and other advan; tage's'particularly from the standpoint of adaptability to diiferent types of distillates as Well as those of diierent boiling ranges.- y

The extraction of Benzene-front hydrocarbon fractions containing the Vsame has been of lincreasing importance in recent years because of continued demands-,for this material' for use as such or as an intermediate Vin 'nu-j merousindustn'al processes, and with the" increased production of reformed naphtha of desirably high aromatics content the need for eilicient methods for separation and recovery of benzene, andof other individual mono-nuclear aromatic hydrocarbons, ,hasV also increased. The improvedV method of the present invention is applicable; for recovery of benzene aswell as tothe recovery ofY toluene.

' The more typical contaminants found associated with .benzene in a closely fractionated benzene cut derived from :a vcatalytically reformed naphtha include methylcyclopentane, which is generally present as the `predominating Ycontarrinating naphthene, and various paratlins, including methylcyclohexane, n-hexane, dimethyl'pentanes and ,methylpentnies An agent having high selectivity .in the Aseparation by extractive distillation of methylcyc'lopentane freni benzene aswell as in the'separation of the usually -occurring C6 and C7 paratins from benzene would obviously be desirablefor use in processes involving benzene recovery.

In accordance with 'the` present invention a closely cut naphthairaction containingA one or more mononuclear aromatic hydrocarbons is subjectedto extractive distillation vin` the presence of pentanedione-2,4V with the separation `of a vapor overhead concentratedin non-aromatic contaminants and a liquid bottoms fraction containing the solvent concentrated with benzene. The system is oper- ICC ated in continuous manner by charging thevsolvent to the upper part of a fractional distillation tower andy bringing in the hydrocarbon feed to be separated or purified near the middle of the tower, The liquid fraction comprising the solvent and aromatics is fed to a stripper ,forseparation and the solvent stripped of benzene recirculated to the extraction tower. v

The operation above described is bas-ed on the principle that the extractive distillation solvent effectively alters the activity coefficient of one or more components of the feed to render the same less or more volatile and since the relative volatility of the contaminating naph-thenes and parafns as compared with benzene (or methylated benzene) is greater in the presence of the solvent, a higher degree of separation is possible than that obtained by ordinary methods of fractional distillation.

In the accompanying drawing there is shown a flow diagram illustrating a practical adaptation of the .invention in a preferred operation.

Pentanedione-ZA is particularly advantageous for use in the extraction of benzene for a number of reasons which, when taken together, contribute in large measure to the eciency of the present process. On the one hand, this solvent has a boiling point sufficiently high to enable ready separation'from benzene by stripping and yet low enough to permit reasonable reboiling temperatures. In addition, lall of the hydrocarbon components encountered in the charge have a fairly high solubility in pentanedione- 2,4 so that the capacity of the system is not unduly limited by the requirement Iof an excessively high solvent concentration in order to maintain only one liquid phase in the trays of the extracting column. Moreover, this solvent has a favorable `eiect on the relative volatilities of the components to be separated. Also, pentanedione-2,4 is quite stable and relatively inert with respect to the components of the charge and is a non-toxic, non-corrosive material which is normally liquid. Since this solvent (B'. P.=l40.5 C.) has a boiling point above that of toluene, it is likewise suitable for the separation of toluene from non-aromatic components of the same approximate boiling range.

The comparative selectivity of a solvent for use in extractive distillation is best determined by its individual behavior with respect to various typical components. This behavior may be expressed as the relative volatility of the non-aromatic constituent to benzene in the presence of the solvent:

volatility of non-aromatic hydrocarbon volatility of benzene Example I number of experimental mixtures containing pentanedione-2,4 with respectively 5% each of methylcyclopentane, benzene, 2,4-dimethylpentane, and n-hexa'nek were made up, and these mixtures individually subjected to distillation, with the following results:

(a) Activity coeilicient (fyy) can'be obtained from the equation Y T 'YzPa/Mz W* PM wherein 1r is the total pressure above the liquid phase, M is the mol fraction of the x and y components respectively, and P is the vapor pressure of these respective pure components at the boiling temperature of the mixture. 7x, the activity coecient of the solvent, is essentially unity because of the high solvent concentration. (b)

Example II A liquid mixture containing 50% benzene and 50% methylcyclopentane .is subjected to fractionation in an extractive distillation column with pentanedione-2,4 as the solvent. The conditions of operation are set so that the solvent comprises 80% of the liquid on the trays. Small samples of liquid are collected from the trays at several points in the extractive distillation section of the column: at the tray where the solvent is introduced, near the middle of the column, and at the bottom of the column. The table below shows the compositions and relative Volatilities at these points:

Liquid Concentration,

Mol. Percent Relative Volatility, M CP/Bz Position in Column Bz MCP PDO 20 10 trace trace Example III A reformate of boiling range 57-85 C. and having the following composition:

Mol percent Benzene 51.0

Parains 31.0 Naphthenes 16.0 Oletins 2.0

is introduced in liquid state at a rate of 100 mols per hour into the mid-section `of a tower, as indicated at 10 in the accompanying flow plan, and is extractively distilled with pentanedione-2,4, the reformate being admitted to an intermediate position of the tower or column through line 11. The pentanedione2,4 enters near the top ofthe tower through line .1.2, at a temperature of about 95 C. From the bottom of the column there is withdrawn through line 13, at about 120 C., a mixture of 50.5 mols benzene, 0.5 mol non-aromatic hydrocarbons, and 204 mols pentanedione-2,4. This bottoms stream is sent to a benzene stripper 14 which separates it into a distillate 1S containing 50.5 mols benzene and 0.5 mol non-aromatic hydrocarbons and a bottoms fraction 16 containing pentanedione2,4 with a trace of benzene. This bottoms stream is cooled from 140 C. to about 95 C. as indicated at 17, and is recycled into the extractive distillation tower 10 as solvent. For starting up the operation, solvent is supplied through branch line 18.

Vapor from the top of the extractive distillation tower,

withdrawn through line 19, has a temperature of about 66 C. and has the following composition:

Mol percent Pentanedione-2,4 trace This vapor is totally condensed at 20 and part of the resulting distillate is returned to the top 'of the tower as reflux 21.

Alternatively, if desired, the naphtha feed may be introduced in vapor or mixed vapor and liquid state, without substantial change otherwise in the operation set out in the foregoing example. The introduction temperature of the pentanedione may be higher or lower than that indicated in the example, depending largely on the eiciency of the column, as between about and 105 C.

In general for the extractive distillation of hydrocarbon fractions it is preferred to employ a proportionate quantity of extractive solvent in the order of 60 to 80% of the total liquid on the column trays.

While the principles of operation hereinbefore described are applicable to the treatment of wider boiling hydrocarbon mixtures it is preferred to subject the naphtha to prefractionation separating out respectively a benzene cut and if desired a toluene cut of about 10 F. spread in boiling range, thereby limiting the required size of the aromatic separation unit and improving the eiciency of performance.

Obviously many modifications and variations of the invention as hereinbefore set forth may be made without Ideparting from the spirit and scope thereof and therefore only such limitations should be imposed as are indicated in the appended claims.

What is claimed is:

1. The method of separating desired aromatic hydrocarbons from a naphtha fraction containing at least one member of the group consisting of benzene and toluene which comprises subjecting such naphtha to extractive distillation in the presence of pentanedione-2,4 solvent, recovering the liquid fraction comprising said solvent containing extracted aromatics, and stripping the aromatics from said solvent.

2. The method according to claim l wherein the said solvent substantially stripped of aromatics is recycled to the extractive distillation step.

3. The method which comprises feeding aromaticscontaining naphtha to an intermediate portion of a fractional distillation tower while solvent consisting essentially of pentanedione-2,4 is charged to the upper part of said tower, withdrawing a vapor overhead rich in non-aromatic hydrocarbons and a bottoms fraction containing said solvent concentrated in aromatics.

4. The method according to claim 3 wherein said naphtha is distillate of a catalytic reformate.

5. The method 4according to claim 4 wherein said distillate is a closely cut fraction comprising benzene.

6. The method which comprises contacting a naphtha reformate fraction, of a boiling range including benzene, with pentanedione-2,4 under temperature conditions to effect removal of a vapor overhead boiling below benzene and the formation of liquid aromatic concentrate rich in and boiling above benzene, distilling the concentrate to recover benzene and returning the distilland to further contacting of naphtha reformate.

References Cited in the tile of this patent UNITED STATES PATENTS 2,325,379 Durrum July 27, 1943 2,413,245 Reed et al Dec. 24, 1946 2,415,192 Rittenhouse Feb. 4, 1947 2,461,993 McKinnis Feb. 15, 1949 

1. THE METHOD OF SEPARATING DESIRED AROMATIC HYDROCARBONS FROM A NAPHTAHA FRACION CONTAINING AT LEAST ONE MEMBER OF THE GROUP CONSISTING OF BENZENE AND TOLUENE WHICH COMPRISES SUBJECTING SUCH NAPHTHA TO EXTRACTIVE DISTILLATIONIN THE PRESENCE OF PENTANEDIONE-2,4 SOLVENT, RECOVERING THE LIQUID FRACTION COMPRISING SAID SOLVENT CONTAINING EXTRACTED AROMATICS, AND STRIPPING THE AROMATICS FROM SAID SOLVENT. 